Adatom mobility for the solid-on-solid model

The relaxation of a crystal surface through surface diffusion is studied within the solid-on-solid model. Two types of (conserved) dynamics are considered. ForArrhenius dynamics we show that the relevant transport coefficient, the adatom mobility, has a simple analytic form: It is independent of orientation, and depends exponentially on the inverse temperature, for any surface dimensionalityd. Together with the expression for the orientation-dependent stiffness this completely determines the macroscopic evolution equation for the surface. The predictions of the macroscopic theory are checked against simulations of profile evolution and roughening ind=1. For one-dimensionalMetropolis dynamics we provide an upper bound on the adatom mobility and obtain numerical estimates of its actual value, which indicate a nontrivial orientation dependence in this case. An alternative derivation of the macroscopic dynamics directly from the master equation is presented and discussed in relation to previous approximate work.Dedicated to Professor H. Wagner on the occasion of his 60th birthday     

Thermal step bunching on the restricted solid-on-solid model with point contact inter-step attractions

We present the restricted solid-on-solid (RSOS) model with the inter-ledge interaction of the point contact type (p-RSOS model). We have made detailed calculation of the Andreev free energy , which is similar to the equilibrium crystal shape (ECS) z=z(x,y), and the surface gradient as the function of the Andreev field . From the calculated and , we have obtained the vicinal surface free energy . The inter-ledge attraction between adjacent steps affects the surface free energies in the equilibrium, and causes the first-order transition on the profile of ECS at low temperature. The inter-ledge attraction also destabilizes the regular train of steps. We also have obtained the thermal step bunching.     

Growth,shrinkage, and stability of interfacial oxide layers between directly bonded silicon wafers

Models for the growth and shrinkage of an interfacial oxide layer and for the stability of the interfacial oxide layer are formulated. Predictions of these models are compared to results obtained by high-resolution transmission electron microscopy. Wafers containing different concentrations of oxygen interstitials are bonded. Depending on the starting concentration of oxygen interstitials in the wafers, the interfacial oxide layer grows or shrinks during long-time annealing at high temperatures. For much shorter annealing times, local disintegration of the oxide layer may occur, which is less severely influenced by the concentration of oxygen interstitials. Rather, it depends on the thickness of the interfacial oxide layer. The influence of rotational misorientation is examined by rotating wafers around their common axes perpendicular to a wafer plane and subsequent bonding. Above a critical angle of about 1–3°, a continuous oxide layer is formed, whereas below this critical angle, sufficiently thin oxide layers disintegrate.Also at Microelectronics Center of North Carolina, USA     

Multiple internal reflection spectroscopy of bonded silicon wafers

Interfaces of bonded hydrophilic and hydrophobic wafer pairs are studied by multiple internal reflection spectroscopy after annealing at 1100°C. Si–H_x and SiO–H stretching modes are still present in bonded hydrophilic wafers. Interfaces of bonded hydrophobic wafers, prepared by joining HF-etched surfaces without defonized water rinsing, are characterized by the dominance of hydrides (SiH, SiH₂, SiH₃). Their concentration is about 100 times higher than for bonded hydrophilic wafers. Comparison with the ATR-spectra of HF-treated surfaces showed appreciable shifts in the peak positions indicating that Si–H bonds might be involved in the bonding process.     

Positron annihilation in synthetic zeolites

Both lifetime and angular correlation of positron annihilation have been measured for a series of synthetic zeolites for which the void structures are known fairly well. All of the zeolites had long lifetime components and a narrow momentum component which are ascribable to o-Ps and p-Ps annihilations in the voids, respectively. The correlation between the width of the p-Ps narrow component and the size of the largest voids showed a remarkable agreement with a theoretical estimate based on the spherical potential well model. The measurement of p-Ps momentum thus appears to be prospective as a tool to determine the size of voids of materials. The lifetime of the long lifetime component, on the other hand, showed a poor correlation with the void size even from a qualitative viewpoint, suggesting that factors other than the simple void size effect are dominant in determining the o-Ps lifetimes. Discussion is made on the cause of the different dependences of o-Ps and p-Ps annihilation parameters on the void size. It has also been found that p-Ps fraction is always larger than one third of the o-Ps fraction in all the zeolites studied. A discussion is presented on this point, too.Formerly, RIISOM     

Atomic,electronic, and vibronic structure of semiconductor surfaces

A brief review on recent progress in the theory of electronic, structural, and vibronic properties of semiconductor surfaces is presented with particular emphasis on the empirical and selfconsistent scattering theoretical method for semiinfinite systems. The current knowledge of the Si(001) (2×1) surface is discussed in detail. The Ge(001) (2×1) surface, as well as, the clean and the Ge-covered GaAs(110) surfaces are addressed, in addition. In the discussion of the results it is shown, that the scattering theoretical method is an extremely versatile tool for calculating electronic surface properties unambiguously with high spectral resolution concerning energy, wavevector, layer-index and orbital type. Currently used approaches for calculating the total energy, Hellmann-Feynman forces and optimal structure models are summarized. Using the total energy as a starting point, the calculation of atomic force constants and surface phonon spectra is exemplified.     

Positron annihilation in synthetic zeolites (II) magnetic quenching effect

Positron lifetime spectra were re-measured for a series of synthetic zeolites using a large time window of observation. Magnetic quenching experiments were also performed for the zeolites, and it has been confirmed that both the ₄ and the ₃ components are due to o-Ps. The annihilation rate of the third component, ₃, showed a good correlation with the size of the largest voids, which is similar to the correlation reported for other compounds. However the annihilation rate of the longest-lived component, ₄, showed a poor correlation with the void size. The ₃ component has thus been assigned to o-Ps in the regular voids of the zeolites, and the ₄ component to that escaped to inter-particle open spaces.     

Surface free energy of laser-produced bond percolators

Pulsed laser irradiation of thin metallic films on non-wetted substrates creates two-dimensional bond percolator structures. We show that these can be understood as frozen minimum free energy configurations. The spacing between nodes is found to depend linearly on the initial film thickness, as verified experimentally for the case of a Ti film on sapphire.     

Transient tunneling spectroscopy (TTS) study of electron tunneling from phosphorus atoms in silicon

Recently developed method of transient tunneling spectroscopy (TTS) is applied to investigate the tunneling dynamics of electrons from phosphorus atoms to the silicon conduction band. In contrast to the conventional constant-current spectroscopic tunneling techniques, in TTS one monitors the evolution of the tunneling process in time. Various difficulties, which may be encountered in the measurements of the tunneling time by TTS, are discussed and illustrated. The temperature dependence of the tunneling time for an isolated phosphorus atom is presented, and possible mechanisms responsible for the decrease of the tunneling time with the lattice temperature T, at T15 K, are discussed.     

Ultrafast carrier dynamics in strained In1−xGaxAs/InP heterostructures

The transfer of electrons and holes from barriers to wells is investigated in strained In_1-xGa_xAs/InP multiple quantum wells by time-resolved luminescence upconversion with 300 fs time resolution. The transfer times are in the range of a few ps and independent of the Ga content. The investigation of Ga-rich structures allows to observe directly the hole transfer.     

Compression of CdCu3Ti4O12 perovskite to 55 GPa

Synchrotron x-ray diffraction measurements of CdCu₃Ti₄O₁₂ (CDCTO) were performed up to 55.5 GPa. There is no structural phase transformation in this pressure range. The irregular curvature shifts of the P-V curve are attributed to the grain surface effect. Analysis indicates that the grain surface of CDCTO is stiffer than the grain interior at higher pressures. We point out that the atoms on grain surfaces must be either densely packed or have a strong correlation with the gain interior in order to have a high dielectric constant, as in CaCu₃Ti₄O₁₂. The derived bulk modulus of CDCTO is approximately 235±7 GPa with K^′=5.1±0.4.     

Modulated photoreflectance characterization of ion-implanted semiconductor wafers

Modulated PhotoReflectance (MPR) measurements on semiconductor wafers implanted with boron or silicon ions in the dose range 5×10¹⁰–5×10¹⁵ ions/cm² are presented. Correspondingly, a one-dimensional theoretical multilayer model is established. In the theory, as the implant dose is lower than a critical value, the variation of the MPR signal is contributed mainly by the implanted defects and damages. However, when the dose is above the critical dose, the change of the MPR signal is chiefly due to the formation and growth of an amorphous layer. The theoretical results are in good agreement with those of experiments.     

Diffusion of silicon in titanium nitride films. Efficiency of TiN barrier layers

Two kinds of reactively evaporated titanium nitride films with columnar (B ₀ films) and fine-grained film structure (B ₊ films) have been examined as diffusion barriers, preventing the silicon diffusion in silicon devices. The silicon diffusion profiles have been investigated by 2 MeV ⁴He⁺ Rutherford backscattering spectrometry (RBS) after annealing at temperatures up to 900° C, in view of application of high-temperature processes. The diffusivity from 400 to 900° C: D (m² s^–1)=2.5×10^–18 exp[–31 kJ/mol/(RT)] in B ₀ layers and D (m² s^–1)=3×10^–19 exp[–26 kJ/mol/(RT) in B ₊ TiN layers. The diffusivities determined correspond to grain boundary diffusion, the difference being due to the different microstructure. The very low diffusivity of silicon in B ₊ TiN layer makes it an excellent high-temperature barrier preventing silicon diffusion.     

Silicide formation by furnace annealing of thin Si films on large-grained Ni substrates

Si films with a thickness of approximately 250 nm have been electron-beam evaporated on thick, large-grained Ni substrates (grain size a few mm to 1 cm in diameter). An in situ sputter cleaning procedure has been used to clean the Ni surface before the Si deposition. Thermal annealings have been performed in a vacuum furnace. Ni₂Si is the first phase that grows at temperatures between 240 °C and 300 °C as a laterally uniform interfacial layer with a diffusion-controlled kinetics. The layer thicknessx follows the growth lawx ²=kt, withk=k ₀ exp(-E _a k _B T), wherek ₀=6.3 × 10^–4cm 2/s andE _a=(1-1±0.1) eV. Because of the virtually infinite supply of Ni, annealing at 800 °C for 130min yields a Ni-based solid solution as the final phase. The results are compared with those reported in the literature on suicide formation by the reaction of a thin Ni film on Si substrates, as well as with those for interfacial phase formation in Ni/Zr bilayers.     

A model for vibrational properties of the atomic interface in A/B fcc metals

The motivation of this theoretical work is to introduce a model calculation for the elastic waves scattering and coherent phonon transport at an atomic nanojunction between face-centered cubic (fcc) leads. The model system A/B consists of two perfect semi-infinite fcc leads A and B, oriented in the same direction and joined by an atomic interface. It is applied to the system Cu/Ni and its inverse Ni/Cu. A theoretical approach based on the matching method is used to study the dynamics of the system A/B.     

Theory of solid solubility for rare earth metal based alloys

Both parabola and ellipse separating schemes are used to study the solid solubilities for the binary alloy systems based on the 13 rare earth metals. It has been found that the soluble elements can be separated from the insoluble ones by a parabolyy ₁=a−bx ₁ ² or an ellipse(x ₂−m)²/c²+(y₂−n)²/d²=1. The results show that the overall reliabilities of the solid solubilities for the 897 binary alloys based on the rare earth metals are 89.2% and 92.8% for the parabola and ellipse regularities respectively. The constants a and b in the parabola equation, andm, n, c andd in the ellipse equation are discussed, which can be related to some appropriate parameters for each host metal respectively.     

Laser ablation of a molten Ga target; comparison of experiments and simulation

The laser ablation mechanism of a molten Ga target was examined through comparison of experiments and simulation. The ablation was performed using an ArF excimer laser (193-nm wavelength and 20-ns duration) in vacuum. The observed existence of the threshold laser fluence (necessary for onset of the ablation) and linear increases in both the Ga deposition rate and the Ga emission (fluorescence) intensity with the laser fluence were satisfactorily explained by the simulation based on the thermal ablation model. In addition, it was found that most of the ablated particles are transferred to the substrate not in the form of an excited ion but an excited neutral atom, except at and near the target. Received: 3 September 2001 / Accepted: 7 November 2001 / Published online: 20 December 2001     

Nanometer-microscopy of the electron transmission through an ultrathin (3–22 nm) Au film and of the Au-Si schottky barrier height

Direct imaging with nanometer scale resolution of the Schottky barrier height and of the ballistic transmission of electrons through an ultrathin metal film is demonstrated for the first time. The images are obtained by applying a new pixel-by-pixel evaluation method to the ballistic electron emission spectroscopy (BEES). We find a laterally uniform Schottky barrier height _B=0.88 eV for ultrathin (3–22 nm) Au films evaporated on Si. The transmission coefficient is strongly correlated with the island structure of the Au film. A transmission decay length =14 nm is determined by a statistical analysis of the transmission coefficient with variation of the film thickness.     

Short pulse UV laser ablation of solid and liquid gallium

Received: 3 August 1998/Accepted: 7 August 1998     

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

We address the role of surfactant adsorbates in determining changes in the homoepitaxial growth mode of metals, discussing the case of Sb on Ag (111). From ab initio calculations, we extract evidence that the mechanism operative in this system is that Sb induces an irregular shape and an increase in density of the growing Ag islands, and an ensuing increase of the number of attempts for an adatom to descend to a lower terrace. This results from a combination of peculiar properties of this system: Sb is adsorbed insubstitutional surface sites, leading to the formation of a Sb–Ag surface alloy; deposited Ag has reduced mobility on Sb-covered Ag (111), from which follows a higher nucleation probability. The island shape is irregular since the surface alloy is disordered. Surface seggregation of Sb once the growing layer is completed furthers the phenomenon for many deposited Ag layers. Our explanation of the surfactant action of Sb on Ag (111) does not require a reduction of the downstep diffusion barrier, which may, however, be a concurrent factor helpful to interlayer mass transport and layerby-layer growth.